Diesel engines, because of the way they operate, emit soot particles or very fine droplets of condensate or a conglomerate of the two (particulates) as well as typical harmful gasoline engine exhausts (i.e., HC and CO). These “particulates” (herein Diesel soot), are rich in condensed, polynuclear hydrocarbons, some of which may be carcinogenic.
As the awareness of the danger Diesel soot presents to health collides with the need for greater fuel efficiency that Diesel engines provide, regulations have been enacted curbing the amount of Diesel soot permitted to be emitted. To meet these challenges, soot filters have been used. The filters have had many configurations as exemplified by GB 1,014,498 and U.S. Pat. No. 4,828,807. The most common and useful filters have tended to be porous ceramic honeycombs that have plugged channels such that the exhaust gas must enter a channel and pass through the wall of the channel as exemplified by U.S. Pat. No. 4,329,162.
To more effectively catalyze the combustion of soot and mediate polluting combustion gases such as NOx, CO and unburnt hydrocarbons, catalysts have been used. Catalysts have been employed on filter elements in series where the gas passes through one filter with a catalyst thereon and then passes through another having a different catalyst such as described by GB 1,014,498 and U.S. Pat. No. 4,828,807. In addition, catalysts have been placed on a flow through substrate honeycomb (three way catalytic converter in one's car) separately in front of a filter such as in U.S. Pat. No. 4,902,487. In a variation of this approach, a catalyst has been placed partially in the channels of a honeycomb filter on the inlet channels walls effectively creating a flow though catalyst on the front portion of the inlet channels as shown by U.S. Pat. Nos. 6,294,141 and 6,753,294. Each of these suffers from reducing the total effective filter area and excessive thermal stresses due to exaggerated different combustion along the length of the filter.
Finally U.S. Pat. No. 4,857,089 describes the use of ceramic layer that may be catalyst carrier particulates (e.g., alumina) or particles of the same composition as the walls of the ceramic honeycomb filter (e.g., cordierite) on the outlet end of the honeycomb filter to mitigate damage from burning of the soot to regenerate (i.e., clean the filter so that the engine can continue to operate).
What is needed is a Diesel particulate filter that avoids one or more problems of the prior art such as one of the aforementioned problems. In particular, it would be desirable to provide a Diesel particulate filter that maximizes the effective filtration area while smoothing out temperature differences within the catalyst due to combustion of differing species along the length of the filter. It would also be desirable when doing so to minimize or even reduce the pressure drop experienced in the filter due to reduction of the pore channel area due to coating with catalysts or other ceramic particulates.